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# Refraction-I.ppt

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# Refraction-I.ppt

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### Refraction-I.ppt

1. 1. What is Refraction • When rays of light traveling through air enter a denser transparent medium, the speed of light is reduced and the light rays proceed at a different angle, i.e., they are refracted. • Except when the rays are normal Refraction in Ophthalmology • Methods for evaluating optical and refractive state of the eye 2
2. 2. Emmetropia • Parallel light rays coming from infinity are focused at the plane of the retina when accomodation is at rest. • Clear image of a distant object formed without any internal adjustment of the optics of the eye. • Absence of emmetropia = Ametropia 3
3. 3. Progress of refractive state of eye • Birth : +2 to +3 D • 90% of children at age 5 yrs are Hypermetropic • 50% of children at age 16 yrs are Hypermetropic • After the period of growth has passed , refractive state tends to remain stationary, until in old age a further tendency of hypermetropia is evident. 4
4. 4. Refractive data in adult • Normal axial length ≈ 24 mm • Change in axial length by 1mm = ±3D • Refraction at corneal surface= +40 to 45(+43)D • Change in Corneal Curvature by 1mm = ±6D • Refraction by unaccomodated lens= +16 to 20(+17)D 5
5. 5. Angle kappa (κ) κ ( F M B N D Optic axis • M = Macula • D= Centre of pupil, on cornea • N = Nodal point κ = “Between the visual axis and pupillary line, hence roughly corresponds to angle α”. 6
6. 6. Anomalies of the optical state of the eye • Myopia • Hypermetropia • Astigmatism 7
7. 7. What is Myopia ? • Diopteric condition of the eye where parallel incident rays from optical infinity focus anterior to light sensitive layers of retina when accomodation is at rest. • K/c/o shortsightedness 8
8. 8. Myopia – Optics Diverging lens Emmetropia 9
9. 9. Optics of Myopic eye • Far point is at a finite distance inversely proportional to the degree of myopia • Weakest concave lens that diverges rays just sufficiently to focus them at the retina is to be used • Poor visual acuity is compensated to some extent by enlarged image size due to the nodal point being further from the retina 10
10. 10. Causes of Myopia • The causes of myopia are not known. • Epidemiological correlation suggest... ▫ Lengthy periods of close work are probably a contributory factor ▫ There is some genetic predisposition to myopia and its severity 11
11. 11. • Axial • Curvature • Index • Positional 12
12. 12. Axial Myopia • AP diameter increased to 25.5 to 32.5 mm • 90-95% cases(commonest) • 1mm=3D • There may be… ▫ pseudoproptosis resulting from the abnormally large anterior segment, ▫ a peripapillary myopic crescent from an exaggerated scleral ring, ▫ posterior staphyloma 13
13. 13. Curvature Myopia • Corneal curvature steeper than average, e.g., keratoconus, • Radius <7-8.5 mm (normal) • 1 mm=6 D • Lens curvature is increased • moderate to severe hyperglycemia (intumescence) lenticonus (anterior/posterior) spasm of accomodation spherophakia 14
14. 14. Index Myopia • Increased index of refraction in early to moderate nuclear sclerotic cataracts in the elderly. • Many people find themselves ultimately able to read without glasses or having gained “second sight.” • Decrease in refractive index of cortex – diabetic myopia 15
15. 15. Positional Myopia • Anterior movement of the lens is often seen after glaucoma surgery and will increase the myopic error in the eye. • Axial myopia of buphthalmos is countered to a large extent due to posterior displacement of lens-iris diaphragm and flattening of the cornea 16
16. 16. GRADING OF MYOPIA • Low myopia(<-3D) • Moderate myopia(-3D to -6D) • High myopia(>-6D) 17
17. 17. • Congenital • Simple or developmental • Pathological or degenerative • Acquired(secondary myopia) –post-traumatic -post-keratitic -drug-induced -pseudomyopia 18
18. 18. Congenital myopia • Since birth • Diagnosed by 2-3 years • Mostly unilateral • Manifests as anisometropia • Child may develop convergent squint in order to preferentially see clear at its far point (10-12 cms) • Associated with cataract,microphthalmos,aniridia,megalocornea, congenital separation of retina. 19
19. 19. Simple Myopia • Rarely present at birth, but often begins to develop as the child grows. • Usually detected by age 8 or 12 years in school vision tests (school myopia) • May increase during years of growth, stabilizing around the mid-teens, usually at about 5 D or less. 20
20. 20. Pathological Myopia • 2-3% population • Increases by as much as 4 D/yr • Usually stabilizes at about age 20 years and frequently results in myopia – 10 to 20 D. • If progress is rapid from age 15-20, likely to reach 20-30 dioptres • Commoner in women, Jews and Japanese • k/c/o degenerative/progressive myopia 21
21. 21. Pathological Myopia-Etiology 22
22. 22. 23
23. 23. Symptoms 1.Defective vision 2.Muscae volitanes • Floating black opacities in front of eyes • Degenerated liquified vitreous 3.Night blindness 24
24. 24. Signs • Prominent eyeball-elongation of eyeball mainly affects posterior pole and surrounding area • Cornea –large • Deep anterior chamber • Large, sluggish pupil 25 Elogation of eyeball posterior to equator
25. 25. Fundus examination • OPTIC DISC -large and pale -temporal edge presents as a characteristic myopic crescent -peripapillary crescent encircling the disc may be present -super traction crescent may be present on nasal side(retina pulled over disc margin) 26
26. 26. 27
27. 27. Fundus examination • DEGENERATIVE CHANGES IN RETINA AND CHOROID -common in progressive myopia -chorioretinal atrophic patches at the macula -Foster-fuch’s spot -Cystoid degeneration-periphery -Lattice degeneration -Total retinal atrophy 28
28. 28. 29
29. 29. Lattice degeneration Figure: 30
30. 30. Lattice, snailtrack, retinoschisis, white without pressure Snailtrack Retinoschisis 31
31. 31. Posterior staphyloma • Ectasia of sclera at posterior pole • Crescentric shadow 2-3 DD temporal to disc, • Sudden kinking of retinal vessels as they dip over the edges, • Gross atrophy 32
32. 32. Peripheral Degenerations Paving stone 33
33. 33. • DEGENERATIVE CHANGES IN VITREOUS -Liquefaction -Vitreous opacities -Posterior vitreous detachment(PVD)-Weiss reflex 34
34. 34. • VISUAL FIELDS -contraction -ring scotoma • ERG –subnormal electroretinogram due to chorioretinal atrophy 35
35. 35. Complications • Retinal detachment • Complicated cataract • Vitreous haemorrhage • Choroidal haemorrhage • Strabismus fixus convergence Horseshoe Tear 36
36. 36. 1.)Optical treatment of myopia Concave lens -Basic rule-minimum acceptance providing maximum vision -Modes of prescribing concave lens ▫ Spectacles ▫ Contact lens 37
37. 37. 38
38. 38. 2.)Refractive surgery A.Cornea based procedure B.Lens based procedure 1.Radial keratotomy(RK) 2.Laser ablation corneal procedures a.Photorefractive keratectomy b.LASIK c.ReLEx d.ICR implantation e.Orthokeratology 1.Refractive lens exchange 2.Phakic refractive lens(PRL) 39
39. 39. Radial keratotomy • Deep radial incisions in peripheral part of cornea leaving the central 4mm optical zone • Incisions on healing-flatten the central cornea ,thus reduces refractive power • Correct low to moderate myopia(-2 to -6 D ) • Disadv-cornea is weakened -uneven healing-irregular astigmatism -glare at night 40
40. 40. Photorefractive keratectomy(PRK) • Central optical zone of anterior corneal stroma is photoablated using EXCIMER laser(193 nm UV flash) to cause flattening of central cornea • Correction for -2 to -6 D of myopia • Disadv-pain and discomfort -post-op recovery is slow -residual corneal haze in centre affects vision -expensive 41
41. 41. LASIK(Laser assisted in-situ keartomileusis) • Flap of 130-160 micron thickness of anterior corneal tissue is raised • Midstromal tissue is ablated directly with an excimer laser beam • Ultimately flattening the cornea 42
42. 42. • Advan- -no post-op pain -early post-op recovery -no risk of perforation during surgery -no residual haze like PRK -effective for correcting myopia of -12D • Disadv- -expensive -requires surgical skills -flap related complications 43
43. 43. • PATIENT SELECTION CRITERIA 1.Patient >20yrs 2.Stable refraction for atleast 12 months 3.Absence of corneal pathology • ABSOLUTE CONTRAINDICATION FOR LASIK 1.Corneal thickness <450 micrometers 2.Presence of ectasia 44
44. 44. Refractive lenticule extraction(ReLEx) • All-femtolaser-vision-correction • Lenticule of corneal stroma is extracted with femtosecond laser • Correct myopia upto -10D 45
45. 45. Intracorneal ring (ICR) implantation • Into the peripheral cornea at approximately 2/3rd stromal depth. • Flattening of central cornea,decreasing myopia. • Advantage-reversible procedure 46
46. 46. Orthokeratology • A non-surgical reversible method of moulding the cornea with overnight wear unique rigid gas permeable contact lenses. • Myopia correction upto -5D • Used in patients below 18 years of age 47
47. 47. Extraction of clear crystalline lens • Fucala’s operation • Myopia of -16 to -18 D in unilateral cases • Clear lens extraction with IOL implantztion of appropriate power is the refractive surgery for myopia of >-12D • Complications-endophthalmitis,after cataract,retinal detachment 48
48. 48. Phakic refractive lens (PRL) • Intraocular contact lens implantation for correction of myopia >-8D • Special type of IOL is implanted in anterior chamber or posterior chamber to natural crystalline lens. • Complications- endophthalimitis,iridocyclitis,cataract,secondary glaucoma 49
49. 49. 3.)GENERAL MEASURES- -Balanced diet rich in vitamins and proteins -Early management of associated debilitating disease -Visual hygiene 4.)LOW VISION AIDS -In patients with progressive myopia with advanced degenerative changes 5.)PROPHYLAXIS -Genetic counselling 50
50. 50. Thank you 51